Many reservoirs in the world are stressed and/or fractured in nature. Characterization of those stresses and/or fractures can help to detect an oilfield. Further, it can help to optimize well placement and increase productivity of existing fields. One way of characterizing stresses and fractures of the subsurface reservoirs is using well log data. But generally well log data gives limited information in a field to a region around the well and therefore has limited spatial resolution. On the other hand, seismic data samples the entire reservoir spatially. Therefore, estimation of stress and fracture parameters using seismic data is an important development.
One way of seismically characterizing stress and fractures is by detecting and analyzing seismic anisotropy. Presence of horizontal stress differential and/or vertical fractures with nearly uniform strike direction makes a medium transversely isotropic with horizontal axis of symmetry (HTI). HTI medium shows simplest form of azimuthal anisotropy with respect to seismic wave propagation. Generally, seismic waves travel faster along the fracture strike direction and slower perpendicular to the strike direction of the fractures in an HTI medium. Utilizing this property, in current practice, travel time and amplitude (separately and/or together) variation of reflected seismic energy with azimuth is used to characterize fracture property from seismic data (both travel time and amplitude).
While reflected energy only samples a small area in the Fresnel zone from the target layer, refraction energy samples significantly more area of the target layer as it travels long distances in the target layer. Therefore, effect of anisotropy is significantly more pronounced on the refracted wave than the reflected wave from the target fractured layer. Hence, one can argue that the refracted energy analysis should provide better understanding of azimuthal seismic anisotropy than reflected energy. The uses of refracted energy to characterize fractures are very uncommon and there is a need to develop a methodology to characterize seismic anisotropy using refracted seismic energy. Theoretically, Landro and Tsvankin (2007), Sil and Sen (2009) show that, some properties of the refracted energy (e.g. critical angle, critical offset etc) vary with azimuth in an azimuthally anisotropic medium. Even though most of the time reflected seismic energy is used to characterize fractures, use of refracted energy to characterize fractures is still uncommon. Previously, refraction travel time variation to determine anisotropy as discussed.
There is a need to develop a methodology to characterize seismic anisotropy using refracted seismic energy.